<!DOCTYPE html>
<html>
    <head>
        <script src="https://unpkg.com/vue@next"></script>
    </head>
    <body>
        <div id="app" class="demo"></div>
        <script>
            const app = Vue.createApp({data(){
                return {count:4}
            }})

            const vm = app.mount('#app')
            document.write(vm.$data.count)
            // document.write("<br>")
            document.write(vm.count)       // => 4
            document.write("<br>")
            // 修改 vm.count 的值也会更新 $data.count
            vm.count = 5
            // document.write(vm.$data.count)
            document.write(vm.count)
            document.write("<br>")
            // 反之亦然
            vm.$data.count = 6
            document.write(vm.count) // => 6
        </script>
    </body>
</html>